The overall objective of this research proposal is to designate the identity and the regional sites of alkylated nucleosides in DNAs. An acutely sensitive and definitve mass spectrometric method (previously developed by the Principal Investigator) for the identification and localization of modified residues in viral DNAs will be applied to DNAs that have been altered by mutagenic and carcinogenic agents. The analytical method employs thermal and electron impact fragmentation of the intact DNA or DNA fragments and yields diagnostic ions which identify unusual purine or pyrimidine bases present in the DNA. Analysis of restriction endonuclease fragments, pyrimidine tracts, and other enzymatic degradation products from the DNAs will contribute to localizing the regions in the whole DNA molecule that contain the alkylated residues.